Pillow block and flanged bearing units are basically self-contained bearing systems which include a housing for containing oil, a shaft sleeve adapted to be secured to a shaft, and a bearing. The two most common types of housed units, pillow block and flanged units, have similar features, but flanged units are mounted by bolts through a housing flange whereas pillow block units include a mounting base. Because of the similarities, both units are sometimes referred to pillow block units and that practice is followed to some extent hereinafter.
Housed units offer a number of advantages including ease of installation and mounting. Moreover, the housings can be mounted in any position and self-aligning and relubricatable designs are available. They provide an acceptable choice in many applications including: conveyor systems, fans and blowers, baggage lines, material handling, food packaging plants and textile machinery. Housed units are well suited for a variety of industrial applications from those involving normal or routine loads as well as adverse applications involving wet and dirty environments.
Typically, a self-contained housed unit of the pillow block or flanged type includes a base or housing, superfinished inner and outer rings, high grade ball bearings and precise ball bearing assembly. Although such units have proven adequate in the past, they are expensive and difficult and relatively difficult to assemble. Many of the problems associated with pillow block bearings are common to all rolling element bearing assemblies and are discussed below.
An alternative to rolling element bearing assemblies is the plain bearing assembly, but there are problems adapting such a unit to a pillow block or self-contained unit. Pillow block bearings are typically either grease or oil filled, with oil generally being preferred. To avoid leakage, however, the oil level must be kept below the shaft line, i.e., one third filled. This presents a problem when a sleeve bearing is used in the pillow block bearing assembly because it is difficult to distribute oil along the entire bearing surface.
The problem of insufficient lubrication is not unique to pillow block bearing arrangements. Indeed, the problem occurs in roller assemblies of the type which include at least two parts which are rotatable relative to one another and some form of bearing supporting the two parts on one another.
Roller assemblies are used in a wide variety of applications. Among the most common applications are those in which the roller assembly is used to provide a rolling path. In a typical rolling path or roller conveyor system, an appropriately spaced series of hundreds of more roller assemblies is used to provide a rolling path or track for goods during assembly, manufacture, or processing. Some such applications require miles of conveyor paths and millions of roller assemblies. Roller assemblies are also commonly used as a wheel. One example of a roller assembly used as a wheel supporting a load for movement is an overhead conveyor system wherein the load is hung from a bracket which is typically rollingly supported by two rollers on opposite ends of an I-beam. Roller assemblies are also used as wheels in carts, tilt trucks and the like.
In the past, the most common form of roller assembly has consisted of a metal shaft supporting a roller bearing which in turn supports a metal, plastic or rubber wheel. A number of problems have been experienced with this conventional roller assembly. Among other things, the conventional assembly wears too quickly, can seize up and break, is relatively expensive and can be quite noisy. These problems all result, at least partially, from the use of a roller bearing in the assembly. The roller bearing is easily the most expensive component of the system. Unless an extremely precise (and expensive) roller bearing is used, the bearing is likely to be noisy. Moreover, even the best roller bearings wear relatively quickly. Excessive wear of the metal can, of course, lead to failure. Also, in many assemblies the material that wears off is the zinc plating which protects the steel shaft from corrosion. The use of roller bearings also complicates the assembly of the roller assembly because it is necessary to secure the roller bearing to both the shaft and the housing member.
Another disadvantage associated with conventional roller bearing type roller assemblies is that the assemblies almost always include metal bearings lubricated with a grease or oil. The use of the metal and grease or oil presents problems in applications where the roller assembly is likely to be subjected to corrosive materials or in food processing facilities where a premium is placed on sanitation.
One attempt to overcome the disadvantages associated with roller bearing type rollers and provide an improved roller is the so called all-plastic roller. One known example uses a self-lubricating plastic such as nylon reinforced with an aramid fiber to increase the stiffness and load bearing capability of the plastic. The relatively movable shaft and housing are in direct contact with one another and no lubricant is provided.
Because of the use of high performance engineering plastics, it was expected that this all-plastic dry roller assembly would offer longer life and reduced maintenance compared to greased metal rollers. However, in practice a number of problems have arisen. For instance, the coefficient of friction in the dry system is too high; this results in excessive power requirements to drive the roller assembly. Finally, without lubricant, the plastic wears too quickly thus limiting the life of the roller assembly. Finally, the plastics have a limited load capability.
Heretofore, there has been a reluctance to attempt to use lubrication in an all-plastic roller assembly or in an assembly having an all plastic housing because of the difficulty of sealing the lubricant within the all-plastic assembly. It was thought that proper sealing would require a complex seal assembly thus increasing the cost of the overall roller assembly. Moreover, there has been no practical way to ensure the integrity of the seal as is required in certain applications such as food processing. In large part, the reluctance to use lubricant results from the recognition that it is extremely difficult, if at all possible, to seal between two relatively movable surfaces when the interface of the two relatively moving surfaces is acting as a fluid dam.
The significance of the above described problems with known roller assemblies can be appreciated when one considers that a large manufacturing or food processing facility can easily include millions of roller assemblies. If the roller assemblies wear too quickly, it becomes necessary to replace each of the millions of roller assemblies. This can cause manufacturing delays and is, of course, very expensive. Moreover, when a single application uses millions of roller assemblies, even a small decrease in the cost of each assembly can drastically reduce the overall cost of a conveyor system. Likewise, any reduction in the cost and time required to assemble individual rollers and conveyor systems using such rollers can be very significant. Further, any noise generated by each individual roller is greatly magnified when millions of roller are used in a single facility.
Thus, there remains a need for a roller assembly which is inexpensive, durable, leak proof, non-corrosive and quiet. Such as assembly, if possible, could be adapted to various applications including among others, plain bearing arrangements suitable for use in a pillow block bearing assembly.